New York — Most people in the world, it's fair to say, want to do a little good. At the very least, we try to follow a kind of secular golden rule: Try to do no harm. But in our communities and around the world, there's a kind of person who takes all this further – to an extreme, even. They're called, most often, "social entrepreneurs," and some of them have become famous, at least in certain circles: Muhammad Yunus, the 2006 Nobel Peace Prize laureate, is revered in do-good financial circles for pioneering microfinance, a lending system for the very poor. Some rub shoulders with the famous: Jody Williams, whose global campaign against land mines won the 1997 Nobel Peace Prize, found a fan in Britain's late Princess Diana. Others are treated like rock stars themselves: Mention Paul Farmer, the public-health innovator and subject of the bestseller "Mountains Beyond Mountains," and grown graduate students swoon like tweens at a boy band concert.

From protecting our natural environment to improving our children's education to combating global poverty and disease, we've come to rely on extreme do-gooders to tackle the world's toughest problems. And they're happy to do so, even though their dedication will cost them in the long run. Few of them will make as much as they could in the private sector. They may lose a relationship with a loved one to their work, or miss their kids' big moments.

All of which raises the obvious: Why? What makes these people tick, and how do they sustain a lifetime of commitment to a change that might take generations to see?

"We call it a moment of obligation," says Lara Galinsky, of the Echoing Green Foundation, whose 471 fellows have raised more than $1 billion for their causes since 1987. "It's usually not a dramatic moment; it's a gathering of moments, but it's very clear. It's when something gathers such force that you can't ignore it."

It doesn't happen that often. There is only 1 social entrepreneur for every 10 million of the rest of us, according to calculations of Ashoka, an organization that funds social entrepreneurs around the world. Ashoka founder Bill Drayton bases his calculations on nearly 30 years' worth of seeking out the elusive combination of vision and passion that social entrepreneurs put into practice.

"The core defining element is that they simply cannot come to rest ... until their dream has become a new pattern across all of society," says Mr. Drayton. "This is very different from everyone else: the scholar or the artist expresses an idea, and they're happy. The manager ... make[s] the company work. The social worker, the professional help people ... make their lives better. None of that would remotely satisfy the social entrepreneur. Their job is to change the system."

Traditionally, we think of spending a lifetime struggling against "the system" as a noble sacrifice: Think Crystal Lee Jordan, who lost her mill job for trying to unionize her colleagues and whose story won Sally Field her first Oscar, as Norma Rae. Think Erin Brockovich, the legal secretary, immortalized by Julia Roberts, who threw her all into a case against a utility company for contamination of a community's water supply.

But in the real world, social entrepreneurs seem to think in terms of everything except sacrifice. "I've interviewed several hundred social entrepreneurs over the last 15 years," says David Bornstein, author of "How to Change the World: Social Entrepreneurs and the Power of New Ideas," "and the thing that struck me is how little they think about sacrifice, and how little their lives are about sacrifice, and how much their lives are about really doing something that gives them extraordinary joy and satisfaction when they're successful."

Whether they're fulfilled visionaries or ascetic volunteers doesn't change the ways in which they make a difference, of course. Changing the system has a lot of prerequisites: A good idea, financial and human resources, and a will of steel, to name a few. But most important may be the often elusive chutzpah.

"You see the world as changeable because you can change it. The vision of self and vision of world are related," says Drayton. "If you come from a background [where] everyone says 'You can't, you can't, you can't' – you won't. You won't have practiced it. You won't have defined yourself that way."

That self-definition can have roots in several places – school, home, social circle – but it may also be especially American. Alexis de Tocqueville famously analyzed the American tendency for helping one's community, and though social scientists like Robert Putnam have alleged that our communitarian impulses are dying off, the United States still fosters a unique form of social engagement. "The unique thing about a voluntary association," says Marshall Ganz, a lecturer at Harvard University's John F. Kennedy School of Government, "is that you don't have the economic or political power to fire people or put them in jail. You have to elicit a voluntary contribution, so that requires a lot more reference to core values."

For every extreme do-gooder, there are probably – thankfully – uncounted others who want to see social change, sharing their talents and time at a pace most of us can handle. There are, Dr. Etzioni says, "millions of people who make a contribution despite the fact that they have many other duties ... [by] going and bringing orphans into our home, doing community service, teaching people to read, being a pal. There are hundreds of ways we do that."